1. Field of the Invention
The present invention generally relates to a solid immersion lens (SIL), a condensing lens using such solid immersion lens, an optical pickup device, an optical recording and reproducing apparatus (or a magneto-optical recording and reproducing apparatus) and a method for forming a solid immersion lens. More particularly, this invention relates to a solid immersion lens, a condensing lens, an optical pickup device, an optical recording and reproducing apparatus and a method for forming a solid immersion lens suitable for use with a so-called near-field optical recording and reproducing system in which information is recorded on and reproduced from an optical recording medium (or a magneto-optical recording medium) by using a condensing lens of which numerical aperture is increased by a material having a large refractive index of an optical lens.
2. Description of the Related Art
Optical recording mediums (including a magneto-optical recording medium) represented by a compact disc (CD), a mini-disc (MD) and a digital versatile disc (DVD) are widely used as storage mediums for storing music information, video information, data, programs and so forth. However, in order to provide high sound quality, high picture quality, longer time recording and playback and a larger storage capacity for music information, video information, data, programs and the like, an optical recording medium having a larger storage capacity and an optical recording and reproducing apparatus (including a magneto-optical recording and reproducing apparatus) capable of recording and reproducing such optical recording medium have been desired so far.
In order to meet such requirements, in the optical recording and reproducing apparatus, a wavelength of light emitted from a light source, for example, a semiconductor laser has been shortened, a numerical aperture of a condensing lens has been increased and a diameter of a beam spot of light focused on an optical recording medium through the condensing lens has been reduced.
For example, with respect to a semiconductor laser, a GaN semiconductor laser of which oscillation wavelength has been reduced from 635 nm, which is an oscillation wavelength of a related-art red laser, to 400 nm band has been put into practice and thereby a diameter of a spot of light has been reduced more. Also, with respect to a semiconductor laser with a shorter oscillation wavelength, a far-ultraviolet solid laser, manufactured by SONY CORPORATION under the trade name of UW-1010, capable of continuously emitting light of a single wavelength of 266 nm and the like are now commercially available on the market and hence a diameter of a beam spot of light has been reduced more. In addition to the above semiconductor lasers, a laser capable of emitting laser light with a wavelength (266 nm band) twice that of an Nd:YAG laser, a laser capable of emitting laser light with a wavelength (202 nm band) twice that of a GaN laser and so on are now under study and development.
A so-called near-field optical recording and reproducing system are now under consideration, in which a condensing lens with a numerical aperture larger than 1, for example, can be realized by using an optical lens such as a solid immersion lens (SIL) with a large numerical aperture and in which the objective surface of this condensing lens can be made close to the optical recording medium with a distance of approximately one-tenth of a wavelength of its light source to thereby record and reproduce information (see cited patent reference 1, for example).
In this near-field optical recording and reproducing system, it is important to hold a distance between the optical recording medium and the condensing lens in an optical contact state with high accuracy. Also, since a diameter of a beam spot of focused light introduced into the condensing lens after it has been emitted from the light source is reduced and a distance between the optical recording medium and the condensing lens is extremely decreased to become less than about several tens of nanometers, an inclination margin between the optical recording medium and the condensing lens, that is, so-called tilt margin becomes very small, and it is unavoidable that the condensing lens is much restricted from a shape standpoint.
FIGS. 1A and 1B of the accompanying drawings are respectively a schematic side view and a schematic plan view showing an arrangement of an example of a solid immersion lens according to the related art. As shown in FIG. 1A, a solid immersion lens 1 is formed as a hemispherical shape of a hyper-hemispherical shape (hyper-hemispherical shape in the illustrated example. A thickness extending along the optical axis is r when the solid immersion lens 1 is formed as the hemispherical shape, and a thickness extending along the optical axis is r (1+1/n) when the solid immersion lens 1 is formed as the hyper-spherical shape. An objective surface 9 is shaped as a flat surface, for example.
Such solid immersion lens 1 and the optical lens can be sequentially disposed from the objective side of the optical recoding medium to thereby construct a near-field condensing lens.
When the condensing lens having the above arrangement is applied to an optical recording and reproducing apparatus, for example, the condensing lens is mounted on an optical pickup device having a biaxial actuator and a distance between the optical recording medium and the condensing lens is maintained in the optical contact state. When the above condensing lens is applied to a magneto-optical recording and reproducing apparatus, a magnetic head device for use in magnetic recording and reproducing is assembled into an optical pickup device and a distance between the optical recording medium and the condensing lens is similarly maintained in the optical contact state.
Also, with respect to the shape of the solid immersion lens, a shape in which an objective surface of a hemispherical or hyper-hemispherical lens is processed as a circular cone shape, a shape in which a magnetic coil is provided around the remaining central portion of the objective surface and so forth have been proposed (see cited patent reference 2, for example).
Cited patent reference 1: Official gazette of Japanese laid-open patent application No. 5-189796
Cited patent reference 2: Official gazette of Japanese laid-open patent application No. 2003-161801
In the above-mentioned near-field optical recording and reproducing system, in order to stably control the condensing lens which is moved in the focusing direction and/or tracking direction of the optical recording medium, it is requested that the lens should be reduced in diameter and that it should be miniaturized. Further, in order to stably record and reproduce the optical recording medium, a tilt margin between the optical recording medium and the condensing lens should be increased.
For example, if the objective surface of the solid immersion lens is formed as a circular cone shape and the tip end portion of its circular cone is processed as a flat surface, then when a distance between the solid immersion lens and the condensing lens is selected to be a very short distance as short as approximately several tens of nanometers, it is possible to maintain a certain amount of the tilt margin between the lens and the optical recording medium.
However, when the solid immersion lens in which the objective side is formed as the circular cone shape is formed, depending upon the inclination angle of the circular cone shape, it is difficult to obtain a solid immersion lens with a large numerical aperture or there is a risk that a holding member for holding the lens will not be bonded to the lens satisfactorily.
Also, when the objective side is formed as the circular cone shape as described above, according to the present process technology, there is a limit in reducing a diameter of a lens.
This defect will be described below. As an ordinary method for forming a solid immersion lens, there is enumerated a method in which a high refractive index material of a cubic shape, for example, is processed as a so-called ball lens of substantially a spherical shape, this ball lens is further processed as a hemispherical or hyper-hemispherical lens and its objective surface of the hemispherical or hyper-hemispherical lens is processed as a circular cone shape.
When a solid immersion lens of a hyper-hemispherical shape with a diameter of approximately 1 mm, for example, is formed by a mechanical polishing or a cutting means according to the above-mentioned method, it is extremely difficult to process its objective surface, that is, the top portion of the circular cone shape as a flat surface with a radius of less than about several tens of microns with high accuracy. While the objective surface with a diameter of about 50 μm can be formed at present, it is difficult to form such objective surface with excellent productivity while the shape of the objective surface can be prevented from being fluctuated.
Further, when a very small flat surface is formed on a part of the objective surface of the lens processed as the circular cone shale by a microminiaturizing process means based on a semiconductor process technology such as an RIE (reactive ion etching), a complex process for masking the circular cone portion by using a photolithography becomes necessary and as a result, a problem arises, in which a process time is extended.
Further, also in this case, if the radius of curvature of the solid immersion lens is as large as about several millimeters, then a problem arises, in which the solid immersion lens is restricted by a tilt margin between the end portion of the objective surface and the optical recording medium.
Further, it is requested that the diameters of respective lenses of the optical system including the solid immersion lens should be further reduced in order to alleviate the load of the weight of the biaxial actuator for mounting the lens thereon to thereby improve servo characteristics such as focusing characteristics, tracking characteristics and reduction of a seek time. In particular, since the solid immersion lens is frequently made of a relatively expensive material that can realize a high refractive index, it is desired that the diameter of the solid immersion lens should be reduced as much as possible.
However, when the radius of curvature of the solid immersion lens is made extremely small, for example, the radius of curvature is selected to be less than the radius of 0.5 mm, for example, a problem arises, in which it becomes more difficult to process, in particular, the tip end portion by the above-mentioned forming method.